1. Field of the Invention
The present invention relates to an image processing method and an apparatus for processing images in accordance with a certain method and, more particularly, to a method and apparatus for performing image processing, for example, in a copying machine, a facsimile machine or a printer to add special information indicating the kind of machine, a serial number and the like, to a recorded image.
2. Description of the Related Art
A method of adding information representing a kind of machine, a serial number, and the like to a formed image has been practiced mainly with respect to image processing apparatuses, such as copying machines, in which an image signal is formed by pulse width modulation and an image is formed by electrophotography, as described in U.S. Pat. No. 5,257,119 and U.S. Ser. No. 9735 (filed on Jan. 27, 1993).
With respect to an apparatus used only for binary image recording and incapable of forming a halftone image, a method has been practiced in which the distance between pixels in a formed image are slightly changed to add information corresponding to the kind of machine and a serial number or the like.
FIG. 9 is a diagram for explaining an information addition method conventionally practiced. This conventional method will be described with respect to the case of pulse-width modulating an image signal originally expressed at a resolution of 400 dpi to form a multivalued image at a resolution of 200 dpi.
According to this conventional art, information is expressed by combinations of marks formed on sixteen lines (marker lines) A0 to A15 disposed at predetermined intervals on a formed image. Information is added to a formed image by forming such combination patterns reiteratively and periodically in the sub scanning direction (recording medium transport direction) on the formed image.
As shown in FIG. 9, marks M having a predetermined size (e.g., 8 (in the main scanning direction: laser light scanning direction).times.4 (in the sub scanning direction) pixels) are reiteratively arranged at certain intervals on each line. Specifically, the number of marks M on line A0 from which one cycle of mark patterns starts is twice the number of marks on the other lines. On this line, pairs of marks M are disposed so that the subsequent one in each pair follows immediately after the preceding one, thereby making it possible to recognize the lines through which one cycle or one set of information is expressed. To accurately decipher information recorded in this manner when the information is read out afterward, an image signal is formed so as to express each mark M in a rectangular form of 32 (8.times.4) pixels at a resolution of 400 dpi.
Information is expressed with such marks M as described below.
This is, as shown in FIG. 9, information is expressed by the distance (L1, L2, L3, . . . ) from the left end of the left one of an adjacent pair of marks M on top line A0 to the left ends of the nearest marks M on the subsequent lines (A1, A2, A3, . . . ) in the main scanning direction.
FIGS. 10(a) and 10(b) are diagrams of an image signal for expressing one mark M conventionally used and a mark M thereby formed. FIG. 10(a) shows the configuration of an image signal for expressing mark M and FIG. 10(b) shows a pattern of mark M formed in an image on a recording medium on the basis of the image signal expressed as shown in FIG. 10(a). As shown in FIG. 10(a), mark M is formed by an image signal forming 8 pixels in the main scanning direction and 4 pixels in the sub scanning direction, i.e., a total of 32 pixels on condition that the resolution is 400 dpi.
With respect to pixels "A" in the total of 32 pixels of the image signal as shown in FIG. 10(a), a predetermined value .beta. level is subtracted from the density value of the original image signal (-.beta.) while with respect to pixels "B" a predetermined value .alpha. level is added to the original image signal (+.alpha.). With respect to the image signal thus modulated, image formation is effected by pulse width modulation so as to have a resolution of 200 dpi such that two pixels in the main scanning direction and one pixel in the sub scanning direction, i.e., two pixels, form one pixel at 200 dpi. A pattern of mark M such as that shown in FIG. 10(b) is thereby obtained if the density value of the original image signal in mark M is uniform through the 32 pixels thereof.
That is, mark M is formed of a set of four recording lines 101 to 104. Recording lines 101, 102, and 104 are thinner (lower in density) than the surrounding ones. Conversely, recording line 103 is thicker (higher in density). Therefore, the above-mentioned distances L1, L2, L3, . . . are obtained as the relationship between the positions of thicker recording lines 103. If the modulation value of the image signal is larger, the resulting marks are easier to read but become conspicuous in an ordinary image. Therefore, optimal values of .alpha. and .beta. are empirically determined.
If the image formed on the recording medium in this manner is read, for example with a scanner having a resolution of 400 dpi, the marks can be correctly read and the read image is analyzed to identify the machine that has formed the image.
However, with the advancement of the image processing technology relating to pulse width modulation and electrophotography, the resolution of image forming apparatuses such as printers, for example, those for forming two-valued images only has been increased to 400 dpi, to 600 dpi, to 800 dpi, . . . In other words, the distance between pixels has become very small, so that it is difficult to decipher added information from formed images.
This problem will be described with respect to the case of forming marks M by an apparatus for forming only two-valued images at a resolution of 600 dpi. If marks M are formed so as to have the same size as that of marks M each having 8 pixels in the main scanning direction and 4 pixels in the sub scanning direction, i.e., a total of 32 pixels at a resolution of 400 dpi as shown in FIG. 10(a) (or 4 pixels in the main scanning direction and 2 pixels in the sub scanning direction, i.e., a total of 8 pixels) by, for example, two-valuing with a modulation signal such as that shown in FIG. 10(a) by an error diffusion method or the like, while preserving the same density, and if .alpha.=40, then the total addition value (total amount of modulation) in the area indicated by "B" in FIG. 10(a) is 40*8=320. In this case, if the image signal is expressed by 8 bits with respect to each pixel (density value: 0 to 255), and if the density value designated by the original image signal of the region to which each mark M is added is "0", the area of "B" has an increase in recording dots relative to the surrounding areas, which is at most about a 1.3 dot (320=255*1.255) with respect to the total amount of modulation. As a result, the essential effect of adding marks M cannot be achieved.
Therefore, there is a need to prepare a modulation signal for 12 pixels in the main scanning direction and 6 pixels in the sub scanning direction, i.e., a total of 72 pixels in order to form marks M, as shown in FIG. 11(a). If the image signal on which this modulation signal is superimposed are two-valued for image formation by an error diffusion method or the like, so that the overall density is preserved, and if .alpha.=40, then the total addition value (total amount of modulation) in the area indicated by "B" in FIG. 11(a) is 40*18=720. In this case, if the image signal is expressed by 8 bits with respect to each pixel (density value: 0 to 255), and if the density value designated by the original image signal of the region to which each mark M is added is "0", two-valued recording dots corresponding to an amount of about 3 dots (720=255*2.823) are newly formed with respect to the total amount of modulation.
However, the positions of three dots 105, 106, and 107 newly formed as three of 18 pixels represented by "B" as shown in FIG. 11(b) cannot be specified because they are changed according to the values of the original signal and the two-valuing method used. Also, a situation may occur in which dots 105, 106, and 107 cannot be discriminated from dots expressing a texture and existing around dots 105, 106, and 107.
As described above, marks M cannot be correctly read from a two-valued image formed at a high resolution when the image is read with a scanner or the like, so that added information cannot be deciphered. Thus, the essential purpose of adding information to an image to identify the apparatus used to form the image has become difficult to attain.
A similar problem is encountered in the case where input image data is multivalued data as well as in the case where input image data is two-valued image data.